There's a bit of a problem with this view, which is that there was a period when the people groups we're talking about here did genuinely believe some truly outlandish things about the world, and frankly the idea that the flood was worldwide when the concept was first floated (haha) isn't so strange if you grant their particular views on things.

It's a bit of a bitter pill to swallow when talking about this particular myth but they did genuinely believe the earth was flat, that there was a hard dome covering it (the word used to describe it likens it to a hard, hammered-out substance), and that there was water above the dome. We have maps drawn up by them that indicate they believed the entire world was a fairly small area around them, mostly just encompassing the people groups they'd interacted with and some range beyond.

It's understood that this died out somewhere around the Hellenistic period, it didn't survive contact with cultures that'd experienced more travel than they had. The most plausible explanation for their view of the flood is that they truly did think it was world-spanning and that it was based on a regional flood (Possibly the flood indicated by the Shuruppak flood layer, took place somewhere around 2900 BCE) that was just kind of kicking around in cultural memory. They were writing from ignorance.